An air firable base metal conductor has been the subject of considerable concern in the art. The unfortunate state of the art is that formulations of necessity frequently relied upon expensive metals such as silver, platinum, palladium, and gold because of their electrical properties and because they are relatively non-oxidizable when air fired. In order to obtain air firable thick film base metal conductors but without resorting to these precious metals, it is necessary to protect such thick film base metal conductors as copper against oxidation during the firing. This protection has taken two courses--(a) air firing within a protective atmosphere utilizing an inert gas such as nitrogen, argon, helium, etc., and (b) blending the base metal with such materials as boron which precludes or tends to suppress the oxidation of the copper during air firing. In both instances there are substantial disadvantages in the described approaches. For example, utilizing thick film conductors for electronic applications in a protective atmosphere necessitates the use of expensive auxiliary equipment thereby reducing the rate of production and entailing expensive monitoring devices for the protective atmosphere ovens. These expedients are part of the conventional art.
In the alternate expedient above described, that of compounding with the copper such antioxidants as boron, for them to be effective, stoichiometric excess is used to fully tie up any available oxygen which might otherwise oxidize the thick film conductor. Residual unreacted boron, being hygroscopic, tends upon aging of the product to change the electrical properties of the thick film copper and thus compromise its electrical properties. At the same time, the boron oxide adulterates the thick film and prevents the proper and effective electrical response which is needed for an optimum conducting function of the thick film. The high cost of the precious metals is a substantial offset to its advantageous physical properties. On the other hand, the non-precious metal compounds such as copper and its equivalents necessitate compounding with it additional materials which, while protecting the product against oxidation, impart undesirable aging properties and undesirable conducting properties to the copper. The other stated alternate to compounding is firing in a protective atmosphere which of necessity entails the use of expensive auxiliary equipment and a substantial number of rejected products. As a result, there is no practical method now in practice for producing a thick film of non-precious conductor material sintered to a non-conductive substrate, and which obviates either the use of a protective atmosphere or the compounding of the conductor with other ingredients which compromise the optimum properties of the copper or other electrically conductive material. Examples of an air firable base metal conductor which utilizes compounding auxiliary materials is found in my previous issued U.S. Pat. No. 4,122,232 issued Oct. 24, 1978 and titled "AIR FIRABLE BASE METAL CONDUCTORS."
There are a substantial number of other precious metal conductors which are not referenced in this disclosure because they are inherently objectionable by reason of their high cost to the extent of being virtually prohibitive except for very specialized applications.